Scientists say this could be playing a role in the dramatic rise of obesity. One-third of U.S. children and teens and more than two thirds of adults are obese or overweight.

"We still have to learn about obesity, including how best to measure it," Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention, said in a written statement.

"However, it's clear that being obese is not healthy - it increases the risk of diabetes, heart disease, cancer, and many other health problems," he said. "Small, sustainable increases in physical activity and improvements in nutrition can lead to significant health improvements."